While Rush will always be a team effort, from what I've read, I think Geddy writes a lot of the
"skeletons" of the songs musically. The basic chord progressions and melodies. Even a lot of the riffs.
Alex recalled once that Geddy came up with the Closer To The Heart guitar intro.
In a 1984 Keyboard Magazine interview - Geddy recalled composing Trees and Subdivisions entirely on keyboard as well as the intro to the song "A Farewell To Kings" - which Alex obviously created his own guitar arrangement for.
On the other hand, I once heard a bootleg of a soundcheck circa Hemispheres Tour, and Alex is playing
the intro riff to "Freewill"! So I would think that was his riff, and an important one at that.
If anyone has good info on riffs/chord progressions etc that were definitely Alex's I'd love to know myself.